There are a number of different file formats for CD-Adapco Star CCM+ meshes. The <filename>.sim file is in a propietary format and at this stage cannot be converted. The <filename>.ccm can be converted using

UGRID is a NASA format for meshes (ref). You must follow the naming convention (ref) for UGRID files, which describes the format and endian-ness of the file data (“ascii8”, “b8”, “lb8”, or “r8”) depending upon which language (C or FORTRAN) was used to create the file. The mapbc file is optional and if not provided the boundary conditions of the zones default to wall.

Tau is a CFD solver produced by the DLR (http://tau.dlr.de/startseite/). Note that Tau is a node-based solver so flow field variables are calculated for and stored at the mesh nodes (or vertices) rather than at cell centres (the approach used by Star CCM+, OpenFOAM, CFX and zCFD). Thus simply converting the file format will not guarantee a good result. In many cases, the conversion will not result in a valid cell-centred zCFD mesh.